Shabdakalpadruma testifies both forms.

कुष्यते आकृष्यते आय- स्थानेभ्यः कोषः । कुष ग निष्कर्षे घञ् कोषो मूर्द्धन्यान्तः तालव्यान्त इत्यन्ये । इति भरतः । 

On Tue, 12 May 2020, 03:09 Dominik Wujastyk via INDOLOGY, <indology@list.indology.info> wrote:
I always thought the first, with ś, was correct, and the second was just an orthographic error.  Then I saw "-koṣa" on the title page of Ingalls's Subhāṣitaratnakoṣa.  He must have thought about this.

So which is "right" and why?

Best,
Dominik

PS I haven't even done elementary due diligence on this question, beyond MW.

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